Cvx is giving error when i use this inequality constraint,why?

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norm(x)>=constant is not a convex constraint. The fact that your constraint is trivial because constant=0 is a coincidence.

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i thought all norms are convex

convex_expression <= constraint is a convex constraint. convex_expression >= 0 is non-convex.

Also, norm(affine_expression) is convex, but norm(non_affine_expression) is not necessarily.

Please read http://cvxr.com/cvx/doc/basics.html#constraints and if you have the energy, read and work exercises in https://web.stanford.edu/~boyd/cvxbook/